Quartz LC 0139 is a digital wristwatch released by Seiko in 1977.
I introduced it before Quartz LC 0124 (released in 1975) compared to this model, I feel that this model is slightly closer to a modern digital watch in terms of design.
However, since this model is one of the earliest digital watches made about 45 years ago, it is a so-called "crown digital wristwatch" in which all basic operations are performed using the crown.

I also have a silver model of this model, but I have acquired a dead stock gold model, so I would like to introduce this one.
Visibility is considerably worse than the silver model, but this is because the color of the reflector behind the LCD is gold to match the body color.(Although my photography skills may be somewhat immature.)
I really admire the developer's commitment to prioritizing color uniformity even at the expense of visibility.

There is one button on the top right, but this button is only used to switch between "Normal display" and "Zone II (so-called dual time) display."
The "date ⇄ seconds" display on the top right can also be switched by turning the crown.
Furthermore, the light (small bulb) is not turned on by pushing a button, but by pushing the crown (pushing it further from the normal position).
Somehow, I feel the strong will of the people of the time who said, "All basic operations for a wristwatch should be done with the crown!"

Zone II is a so-called dual time function, but unlike the dual time function of recent digital watches, it can be set completely independently of the normal display, including the date/day of the week (with the exception of seconds).
Also, in normal display mode, it is always displayed as 12H, and in Zone II, it is always displayed as 24H, so if you set the same time, you can use it to switch between 12H and 24H display.

Just like Quartz 0531 (released in 1975) I introduced before, this model displays the date but does not show AM/PM; as a result, you cannot tell whether it is AM or PM during normal operation, but it features a practical design where AM/PM is displayed only when setting the time.
I suppose what they're saying is, “If you find it inconvenient not to have AM/PM displayed in the standard view (though I don't think most people would mind much if it weren't there), please use Zone II.”

Also, this model has a "low battery notification function" that causes the entire display to blink when the battery is low, but the blinking period is said to be about one week (the battery runs out about a week after the blinking starts), so I have never seen it blinking yet.
Did people back then rush to the watch shop as soon as the LCD display flashed?